1. Field of the Invention
The present invention is directed to beam-hardening correction in an X-ray computer tomography apparatus.
2. Description of the Prior Art
In X-ray computed tomography, a shift of the average energy of the X-rays toward higher values occurs as a consequence of the polychromatic spectrum of the radiation emitted by the X-ray source and as a consequence of the energy-dependent absorption of the X-rays in the body of the patient under examination. This effect is called beam hardening. This effect becomes more pronounced as the transirradiated path in the body becomes longer. In the reconstructed image of the transirradiated body slice, this beam-hardening effect leads to unwanted image artifacts that negatively affect the precise medical interpretation of the image.
Standard algorithms (for instance, polynomial correction) are known for the correction of such image artifacts caused by beam hardening. These produce satisfactory results as long as the spectral absorption or attenuation behavior of the transirradiated body substances does not significantly differ from the spectral attenuation behavior of a reference substance for which the correction algorithm was developed. Water is used as the reference substance in the standard case since water exhibits a spectral attenuation behavior comparable to soft tissue in the human body and the human body is largely composed of soft tissue. Beam-hardening errors then can be eliminated to a significant extent in body regions where essentially only soft tissue is encountered. When, however, the X-rays also passes through bone tissue, the algorithm is no longer accurate since bone tissue exhibits a spectral attenuation behavior that deviates substantially from water. The same is also true, for example, of vessels filled with contrast agent. Since the extent to which soft tissue and bone tissue were responsible for the beam attenuation is initially unknown for the measured values acquired in the course of the examination of a patient, a satisfactory beam hardening correction is not possible based solely on knowledge of the measured values.
Methods referred to as retrospective correction methods were therefore developed wherein an overall image of the transirradiated body slice is first reconstructed from the measured, overall attenuation values, and this overall image is subsequently analyzed and resolved into sub-images. Each of the sub-images shows only a part of the various body substances. In the standard case, a bone image and a soft tissue image are generated. Partial attenuation values that indicate the beam attenuation by the appertaining part of the body substances, i.e., for example, bone tissue or soft tissue, are then calculated from the individual sub-images by re-projection. Subsequently, correction values that are added to the originally measured overall attenuation values are determined for the partial attenuation values of each sub-image. For example, the correction values are taken from correction characteristics that were separately determined in advance for the respective body substances on the basis of reference materials with comparable attenuation. An overall imagexe2x80x94which is now corrected for beam hardeningxe2x80x94of the transirradiated body slice is reconstructed a second time from the corrected, overall attenuation values.
More detailed information about retrospective (post construction) correction methods may be found, for example, in xe2x80x9cA Comparative Study of two Postreconstruction Beam Hardening Correction Methodsxe2x80x9d by G. T. Herman, S. S. Trivedi, IEEE Transactions on Medical Imaging, MI-2, 1983, pp. 128 ff., and in xe2x80x9cA Method for Correcting Bone Induced Artifacts in Computer Tomography Scannersxe2x80x9d by P. M. Joseph, R. D. Spital, Journal of Computer Assisted Tomography, No. 2, 1978, pp. 100 ff.
It has been shown in practice that the known retrospective correction methods can in fact clearly reduce image artifacts caused by beam hardening compared to traditional, standard algorithms, however, image artifacts continue to be observed and elimination or at least reduction thereof is desirable.
An object of the present invention is to provide a computed tomography apparatus with improved beam-hardening correction.
In a first version of the solution, this object is achieved in an X-ray computed tomography apparatus constructed and operating as follows.
A radiator/detector arrangement supplies a set of measured intensity values for each X-ray projection of a body slice of a patient under examination, each measured value thereof being representative of the intensity of the X-rays that have passed through the body slice in a respective partial projection region of the overall projection region. An electronic evaluation and reconstruction unit is connected to the radiator/detector arrangement and is configured for
a) determining an overall attenuation value for each measured intensity value, the overall attenuation value being representative of the actual overall attenuation of the X-rays produced in the body slice in the appertaining partial projection region;
b) reconstructing an overall image of the body slice proceeding from the overall attenuation values;
c) extracting a first partial image from this overall image wherein essentially only those image parts of the overall image are contained that correspond to a first part of the various substances occurring in the body slice;
d) determining respective first attenuation partial values allocated respectively to each overall attenuation value on the basis of this first partial image, the first attenuation partial value being a criterion for the attenuation that the X-rays experiences in the respective projection partial region due to the first part of the substances;
e) extracting a second partial image from the overall image of the body slice wherein essentially only those image parts of the overall image are contained that correspond to a second part of the substances in the body slice differing from the first part;
f) determining respective, second attenuation partial values on the basis of this second partial image allocated respectively to each overall attenuation value, the second attenuation sub-value being a criterion for the attenuation that the X-radiation experiences in the respective projection partial region due to the second part of the substances;
g) determining a correction value for every overall attenuation value on the basis of previously determined beam-hardening correction information stored in the evaluation and reconstruction unit and dependent on the two attenuation partial values; and
h) determining an overall attenuation value corrected for beam hardening for each overall attenuation value according to the following equation:
gcorr=g+k(t1, t2)xe2x80x83xe2x80x83(1),
wherein g is the overall attenuation value, gcorr is the overall attenuation value corrected for beam hardening, t1 is the first attenuation partial value, t2 is the second attenuation partial value and k(t1, t2) is the correction value dependent on t1 and t2.
For determining the beam hardening correction information, in accordance with the invention a set of reference overall attenuation values gref(s1, s2) is determined for a material combination of a first reference material and a second reference material different therefrom. This set of reference overall attenuation values gref(s1, s2) is representative of the actual overall attenuation of the X-rays produced by this material combination at various respective thicknesses of the first material and the second reference material. For this determination, s1 references a first individual attenuation value that is representative of the theoretical linear attenuation of the X-rays by the first reference material for the respective thickness of the first reference material, and s2 references a second individual attenuation value that is representative of the theoretical linear attenuation of the X-rays by the second reference material for the respective thickness of the second reference material. The evaluation and reconstruction unit determines (and uses) the aforementioned correction according to the following equation:
k(t1, t2)=t1+t2xe2x88x92gref(s1=t1, s2=t2)xe2x80x83xe2x80x83(2).
In an alternative, second version, the inventive X-ray computed tomography apparatus is constructed and operates as follows:
A radiator/detector arrangement supplies a set of measured intensity values for each X-ray projection of a body slice of a patient under examination, each measured value thereof being representative of the intensity of the X-rays that have passed through the body slice in a respective partial projection region of the overall projection region. An electronic evaluation and reconstruction unit is connected to the radiator/detector arrangement and is configured for
a) determining an overall attenuation value for each measured intensity value, this overall attenuation value being representative of the actual overall attenuation of the X-rays produced in the body slice in the appertaining partial projection region;
b) reconstructing an overall image of the body slice proceeding from the overall attenuation values;
c) extracting a partial image from this overall image wherein essentially only those image parts of the overall image are contained that correspond to a first part of the various substances occurring in the body slice;
d) determining respective attenuation partial values allocated respectively to each overall attenuation value on the basis of this partial image, the attenuation partial values being a criterion for the attenuation that the X-rays experiences in the respective projection partial region due to the first part of the substances;
e) determining a correction value for every overall attenuation value on the basis of previously determined beam-hardening correction information stored in the evaluation and reconstruction unit and dependent on the respective attenuation sub-value; and
f) determining an overall attenuation value corrected for beam hardening for each overall attenuation value according to the following equation:
gcorr=g+k(t)xe2x80x83xe2x80x83(3),
wherein g is the overall attenuation value, gcorr is the overall attenuation value corrected for beam hardening, and k(t) is the correction value dependent on t.
For determining the beam hardening correction information in the second version of the invention, a set of reference overall attenuation values gref(s1, s2) is determined for a material combination of a first reference material and a second reference material different therefrom. This set of reference overall attenuation values gref(s1, s2) is representative of the actual overall attenuation of the X-rays produced by this material combination at various respective thicknesses of the first material and the second reference material. For this determination s1 is a first individual attenuation value that is representative of the theoretical linear attenuation of the X-rays by the first reference material for the respective thickness of the first reference material, and s2 references a second individual attenuation value that is representative of the theoretical linear attenuation of the X-rays by the second reference material for the respective thickness of the second reference material. The evaluation and reconstruction unit is configured for determining (and using) the overall attenuation value dependent on the reference overall attenuation values according to the following equation applies:
k(g, t)=t+s2xe2x88x92gref(s1=t, s2)xe2x80x83xe2x80x83(4),
wherein
gref(s1=t, s2)=gxe2x80x83xe2x80x83(5)
applies for gref(s1=t, s2).
The two versions have in common the use of a correction value that takes the attenuation by a combination of two different materials into consideration. It has been shown in the human body that the beam hardening by one substance (for instance, bone tissue) is not independent of whether other substances (for instance, soft tissue) are additionally present in the beam path. However, the known retrospective correction methods are based precisely on the premise that this precondition of independency exits, by taking only the attenuation by a single (generalized) substance into consideration. By employing a correction value dependent on the attenuation of two materials, it is possible to come very close to the actual conditions in the human body. Images that are very low in disturbing image artifacts thus can be generated, particularly given exposures of body regions having a comparatively high proportion of bone.
Materials whose spectral attenuation behavior is similar to the body substances that are to be taken into consideration in the partial images are expediently selected as the reference materials. For a partial image that should essentially show only soft tissue, it is expedient to select water as reference material. For a partial image that should essentially show only bone substance, for example, a mixture of K2HPO4 and water can be selected as reference material (S.C.E. Cann, Radiology 166, pp. 509-522 (1988)).